The present invention relates a hydraulic control system for a belt drive continuously-variable speed transmission which includes a belt drive continuously-variable speed transmission means having a primary variable-pitch pulley, a secondary variable-pitch pulley and an endless belt extended between the pulleys, and more particularly to a hydraulic control system which ensures smooth and reliable operation of the belt drive continuously-variable speed transmission means, by applying a pressure factor commensurate with the rotational speed ratio between the pulleys to the hydraulic circuit for controlling the change-speed ratio of the belt drive continuously-variable speed transmission means. The belt drive continuously-variable speed transmission means, hereinafter referred to as "belt drive transmission means", generally comprises an input shaft and an output shaft disposed in parallel relationship to each other, a primary variable-pitch pulley and a secondary variable-pitch pulley mounted on the input shaft and the output shaft, respectively, and an endless belt extended between the primary and the secondary variable-pitch pulleys, each of the primary and secondary variable-pitch pulleys having a fixed pulley flange concentrically fixed to the corresponding shaft and a movable pulley flange adapted to be slidable on the corresponding shaft in the axial direction thereof through the actuation of a hydraulic servomechanism whereby the continuously-variable and stepless rotational speed change is attained by changing the effective diameter of the contacting point of the endless belt with the primary and the secondary variable-pitch pulleys.
A belt drive transmission means, comprising two variable-pitch pulleys and an endless belt extended between the two variable-pitch pulleys, functions to change the change-speed ratio (the torque ratio) on the basis of the torque of the output shaft of the prime mover (the input shaft of the transmission) and the required torque of the driven shaft (the output shaft of the transmission). Such a belt drive transmission means has been employed in small motor vehicles equipped with an engine of low output capacity, for example, as the transmission of motor scooters and motor cycles. Recently, an endless belt formed by linking metallic blocks of a substantially trapezoidal cross section with metallic belts has been developed, which contributes to the increase in the strength and the durability of the endless belt for a belt drive transmission means. Accordingly, trial has been made to apply the belt drive transmission means having such a metallic endless belt to a transmission for a motor vehicle equipped with an engine of a large output capacity.